Reflexes

• Reflex= a rapid, automatic, involuntary response to a stimulus.

  Provide survival advantage.
• Reflexes are rapid because:

  • Fixed pattern response, the response is always the same
  • Not learnt
  • Aids survival
• Reflex arc:

  1. Receptor detects stimulus and creates an action potential in the sensory neurones.
  2. Sensory neurones carries the impulse to the spinal cord.
  3. Relay neurone connects the sensory neurones to the motor neuron within the brain or spinal cord.
  4. Motor neurones carries the impulse to the effector which carries out the response.
• Knee-jerk reflex

  =Spinal reflex, neural circuit only goes up to the spine.

  • When the patella is tapped it stretched the patella tendon and acts as a stimulus.
  • Stimulus initiates a reflex arc that causes exterior muscle on top of the thigh to contract.
  • At the same time a relay neurone inhibits the motor neurones of the flexor muscle causing it to relax.
  • This contraction, coordinated with the relaxation of the antagonist flexor hamstring muscle, causing the leg to kick.
• The absence of the knee-jerk reflex may indicate nervous problems and multiple oscillation of the leg may be a sign of a cerebellar disease.

  The reflex is used to maintain balance and posture.
• Blinking reflex

  • Occurs when the cornea is stimulated
  • Purpose= keep the cornea safe from damage due to foreign bodies such as dust or flying insects entering the eye.
  • Also occurs when sounds are greater than 40-60dB are heard or a very bright light.
  • Blinking as a result of a very bright light (protect lens and retina) is known as optical reflex.
  • Blinking reflex is a cranial reflex.
• Blinking reflex process

  When cornea is irritated, the stimulus triggers an impulse along a sensory neurone. It then passes through a relay neurone in lower brain stem.

  Impulses are then sent along branches of motor neurones to initiate a motor response to close eyelids.

  The reflex initiates a consensual response- both eyelids close in response.
• • Doctors test for blinking reflex when the patient is unconscious.
  • If reflex is present, it indicates that the lower brain stem is functioning.
  • Used to determine whether a patient is brain dead.
• Survival importance

  • Involuntary response= decision making regions of brain aren't involved, so brain can deal with more complex responses. It prevents the brain from being overloaded with situations in which the response is the same.
  • Not having to be learnt= present at birth so provides immediate protection.
  • Fast= reflexes are short, normally only involves one or two synapses.
  • Many reflexes are what we consider everyday actions. Eg: keeping us upright, digestion